The present invention relates to improved methods for manufacturing photovoltaic solar cells and particularly to improvements which result in a back contact structure which increases cell efficiency.
The most relevant reference known to the applicants is U.S. Pat. No. 4,158,591 issued to Avery et al. on June 19, 1979, which patent is hereby incorporated by reference for all purposes. The Avery patent teaches the desirability of forming a shallow p-n junction, the steps involved in manufacturing such a device, and various problems related thereto. In general, the Avery reference is directed toward simplifying the manufacturing process to thereby reduce the final cost of the manufactured devices. One of the primary simplifications taught by Avery involves the destruction of back face p-n junctions which may be formed either intentionally or accidentally during the formation of the front face p-n junction. The back face junction is a reverse biased diode junction which would interfere with flow of current from the active front face junction to the back surface contact. Instead of using a separate step, such as etching, to remove the back surface junction, Avery teaches the use of a back surface metallization containing a material, such as aluminum, which upon firing will alloy through the junction to make ohmic contact with the bulk material of the semiconductor wafer. Avery teaches the use of a continuous back contact metallization which covers substantially the entire back surface of the wafer, thereby destroying substantially all of the back surface p-n junction.
The use of a continuous or full coverage back contact has been standard practice because of resistance considerations. Typically, the bulk material of the semiconductor wafer has a higher resistivity than the thin diffused front layer. Therefore there would be far more loss occurring if current had to travel laterally through the bulk material to reach the rear contact than occurs on the front surface contacts to the thin diffused layer which must be in the form of some type of grid pattern to allow the majority of the light to reach the active junction.
More recently it has been determined that the use of a gridded back contact would lower the operating temperature of the solar cells and thereby improve their efficiency. This is because infrared radiation tends to pass through the solar cell and either be absorbed by a full back surface contact or reflected back through the device. In either case, the absorption of infrared radiation results in higher cell operating temperature which reduces cell efficiency. By proper design of a gridded back contact, it has been found that a net efficiency increase can be achieved through operating temperature reduction even when back contact resistance is taken into account.
The use of a gridded back contact metallization has several effects on the cost of manufacturing solar cells. The gridded contact uses much less metal paste than the solid back contact and therefore significally reduces material costs. However, when a gridded back contact has been used, additional steps have been employed to insure that no reverse junction exists on the back surface of the semiconductor wafer prior to the metallization step. The above referenced Avery patent discusses various prior art techniques used to avoid such reverse junctions. The junction destruction technique taught by Avery has not been thought applicable to the gridded back configuration since only that small part of the reverse junction area covered by the gridded contact pattern would be destroyed upon the firing operation.